Servers base64-encode “grpc-status-details-bin” header with padding, but should be unpadded

The spec for gRPC states that implementations should emit binary headers using *unpadded* base64-encoding (emphasis mine):

> Note that HTTP2 does not allow arbitrary octet sequences for header values so binary header values must be encoded using Base64 as per https://tools.ietf.org/html/rfc4648#section-4. Implementations MUST accept padded and un-padded values ***and should emit un-padded values***.
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However, the server in this repo emits a “grpc-status-details-bin” header (a special binary header containing error details that accompany the gRPC status) that uses padding.